In optical pattern and character recognition systems where microscopic lens systems are required for directing a beam from a flying spot scanner at an extremely small object to be examined, it has been found that there exists the need for automatic focusing systems which enable the microscopic lens assembly to bring the object into optimum focus. For example, in optical pattern recognition systems which are utilized for the purpose of providing a differential white cell count of the blood of a patient, a sample of the whole blood is smeared and dried on a slide and a strain is used to enhance the contrast. In order to make an automatic differential white cell count of the blood, the slide is then used in combination with a microscopic lens assembly in an optical pattern recognition system.
After the slide has been placed in the microscopic instrument, it is then necessary to bring the slide into focus prior to starting the pattern recognition system. However, after the initial white cell is examined and the slide is moved so that the next white cell can be examined, quite often the next white cell is out of focus. It should be noted that when the human eye is used, the eye can tolerate a slightly out of focus microscope because of the fact that the eye can adjust. However, in an automatic system such as a character or pattern recognition system, the microscopic lens assembly must be in substantially exact focus because the circuitry responsive to the light focused through the lens system is not as adaptable as the human eye.
In view of the fact that substantially imperceptible flaws in a slide can require a change of focus of the optical instrument, it is necessary that the pattern recognition system utilized in a white blood cell differential count be constantly focused after each white cell is captured within the field of view of the microscopic lens assembly.
In the absence of an automatic focusing system, after the first cell has been analyzed by a pattern recognition system, each succeeding cell would have to be examined with the probability that the microscopic lens assembly had shifted out of focus. The disadvantage of course is that a cell not in optimum focus of the microscopic lens assembly is not as easy to recognize because of the fact that the transitions between the nucleus and cytoplasm of a white cell and the cytoplasm and the clear portions of the smear of the cell become more gradual as the distance from optimum focus increases. Thus, the recognition of the nucleus portion and the cytoplasm of the white cells is greatly diminished as a result of the blurred images caused by an optical instrument out of focus.